You're correct that Duncan has been the best player on what is consistently one of the best teams. However, look at where that has landed him in the MVP the last 3 seasons.
This past season, the Spurs won 58 games (3rd best in the league), and Duncan averaged 20 PPG and 10.6 RPG. Duncan finished 4th in MVP voting and received no 1st place votes. Kobe Bryant barely made the playoffs, and he receieved 2 1st place votes and came in 3rd.
In 2006, the Spurs won 63 games (2nd best in the league), and Duncan averaged 18.6 PPG and 11 RPG (although he had the foot issues). He finished a distant 8th in the MVP voting.
In 2005, the Spurs won 59 games (tied for 2nd best in the league), and Duncan averaged 20.3 PPG and 11.1 RPG (he missed a few games, though). He finished in 4th and received 1 1st place vote.
Back when he won MVP in 2002 and 2003, he averaged 25.5 PPG and 12.7 RPG and 23.3 PPG and 12.9 RPG. He also averaged 40.6 and 39.3 MPG those 2 seasons. When he came in a distant 2nd in 2004, he averaged 36.6 MPG, 22.3 PPG and 12.4 RPG.
The last 3 seasons, however, Duncan has averaged 33.4, 34.8 and 34.1 MPG and his numbers have dropped consequently, as well as, perhaps, due to age and more help from teammates.
Duncan is 31 years old now, and while he'll surely still be the best player on one of the best teams--arguably even the best player in the league on the best team in the league--I don't see him improving significantly statistically, nor do I see him being higher than I projected in the MVP race.
That's my objective opinion in evaluating the MVP voters habits. It's not my personal opinion on where Duncan deserves to rank in the MVP race.
I've got to admit, I made my post without doing ANY research. I need to read before I type
I have no other choice other than Lebron James. Seems its his turn. He's come up short in the MVP races. Barring injury he will win it imo.
I leave out Kobe Bryant is someone may feel "According to that logic wouldnt it be Kobe's turn?" and it would be a good counter. However I leave him off because the team around him and the conference in which that team plays. Bron and the Cavs are in the East and have a good enough shot having the best record in the East. It would weigh more than what L.A would finish in the standings. I just dont see the Lakers finishing the West with a top 3-4 record.